The present invention relates to a flash light control circuit that controls a quantity of light emitted from a flash bulb so that an optimum exposure may be obtained from a photographic object and others in the course of photographing by means of a camera.
FIG. 4 represents a block diagram showing a flash light control circuit that controls a quantity of emitted light by regulating the discharging time of a flash bulb using an ordinary non-self-hold type semiconductor switching element.
Flash light control circuit 100 is composed of power source circuit A, oscillated voltage boosting means B that oscillates and boosts voltage generated from power source circuit A and thereby charges main capacitor 11, flash bulb 10 that flashes when electric charges charged in aforesaid main capacitor 11 are discharged, main capacitor 11 that holds energy for aforesaid discharge, resistor element 12, trigger circuit C that impresses trigger voltage on aforesaid flash bulb 10 and non-self-hold type semiconductor switching element such as IGBT (hereinafter referred to as switching element) 16 that starts aforesaid trigger circuit C and controls light emission of flash bulb 10. When one terminal of flash bulb 10 and one terminal of a coil positioned at the primary side of a trigger transformer in trigger circuit C are connected electrically to a collector terminal of aforesaid switching element 16 and thus the switching element 16 is turned on and turned off, a period of time for discharging energy stored in aforesaid main capacitor 11 into flash bulb 10 is regulated and thereby a quantity of emitted light is controlled and also the control is made so that terminal voltage V CM on main capacitor 11 may cause the flash bulb to flash a number of times.
FIG. 5 represents a graph showing actions of the flash light control circuit made in the occasion that flash bulb 10 flashes a number of times.
In the figure, pulse signals to be impressed on a gate terminal of switching element 16 are shown in (a) and variations of terminal voltage V CM on main capacitor 11 are shown in (b). Each hatched area in (b) corresponds approximately to the quantity of emitted light.
In aforesaid flash light control circuit 100, however, when one terminal of flash bulb 10 and an input terminal of the trigger circuit are electrically connected to a collector terminal of switching element 16, and flash bulb 10 is caused to flash partially or on a split flashing basis for a certain frequency by turning on and turning off switching element 16 for the frequency identical to aforesaid frequency, electric charges charged in main capacitor 11 are discharged each time the flash bulb flashes partially as shown in FIG. 5, resulting in the drop of terminal voltage V CM on main capacitor 11 which voltage drop causes the sharp reduction of the quantity of light emitted from flash bulb 10. Therefore, when photographing with a camera on a multi exposure basis, for example, an exposure level or the exposure amount caused by a photographic object to be photographed decreases gradually with the progress of exposures, which has been a problem.